Perpendicular magnetic recording system

ABSTRACT

To make a soft magnetic underlayer of a double layered perpendicular magnetic recording medium thinner than heretofore while avoiding saturation. Assuming that Tb1 is the thickness of the soft magnetic underlayer of the double layered perpendicular magnetic recording medium, BS2 the saturation flux density of the same, Tm the thickness of a magnetic recording head&#39;s main pole 1 along a track direction in the vicinity of its floating surface, TWW the track width of the same, and BS1 the saturation flux density of the same, then Tb1&lt;(BS1xTmxTWW)/2(BS2x(Tm+TWW)) is satisfied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a perpendicular magnetic recordingsystem using a perpendicular magnetic recording medium (double layeredperpendicular magnetic recording medium) having a soft magneticunderlayer.

2. Description of the Related Prior Art

In perpendicular magnetic recording systems using a double layeredperpendicular magnetic recording medium, magnetic flux extending from amain pole of a recording head follows a magnetic path that runs througha soft magnetic underlayer of the double layered perpendicular recordingmedium to enter an auxiliary pole of a magnetic head and then returns tothe main pole. Conventional perpendicular magnetic recording media haveadopted soft magnetic underlayers of greater design thicknesses so as toavoid magnetic saturation of the soft magnetic underlayers.

FIG. 4 is a diagram explaining the cross section of a path through whichmagnetic flux flows on a conventional model. As shown in FIG. 4, inorder to prevent a soft magnetic underlayer 6 of a perpendicularmagnetic recording medium from being saturated by the magnetic flux froma main pole 1 of a magnetic head, it is considered that the limit of theprevention is determined by both the area of the cross section 11 of themagnetic flux inside the soft magnetic underlayer, through which themagnetic flux extending from a top face 10 of the main pole passes, andthe saturation flux density of the soft magnetic underlayer 6, and it isnecessary to satisfy the following relational expression:

T _(WW) ×T _(b1) ×B _(S2) >T _(WW) ×T _(m) ×B _(S1),

i.e.,

T _(b1) ×B _(S2) >T _(m) ×B _(S1),  (1)

wherein T_(WW) is the track width of the main pole 1, B_(S1) thesaturation flux density of the main pole 1, T_(m) the thickness of themain pole 1, B_(S2) the saturation flux density of the soft magneticunderlayer 6, and T_(b1) the thickness of the soft magnetic underlayer.In view of this, perpendicular magnetic recording media have beenprovided with a thick soft magnetic underlayer.

For example, Japanese Patent Laid-Open Publication No. Hei 10-283624describes a double layered perpendicular magnetic recording mediumhaving a soft magnetic underlayer of 600 nm in thickness.

In conventional ideas, for example, the saturation flux density B_(S) ofa main pole of 1.6 T, the thickness T_(m) of the same of 0.5 μm, and thesaturation flux density B_(S) of a soft magnetic underlayer of 1.2 Tcombine to require, according to the expression (1), the thicknessT_(b1) of the soft magnetic underlayer as great as 0.67 μm or more. Thisis no less than ten times the thickness of the magnetic recording layersof current in-plane magnetic recording media which is no greater thanseveral tens of nanometers. Given here that the growth rates are nearlyequal, the growth time becomes more than ten times, causing a drop inproduction efficiency and a rise in cost. Moreover, the consumption ofthe target used in the sputtering also increases for a cost increase.Besides, greater thicknesses deteriorate surface roughness because ofinhomogeneous grain growth. This causes a problem since high-densitymagnetic recording media require low surface roughness for the sake ofreducing head-medium spacing. Accordingly, conventional double layeredperpendicular magnetic recording media were disadvantageous as comparedwith in-plane magnetic recording media and single layered perpendicularmagnetic recording media.

To avoid the saturation of a soft magnetic underlayer without thickeningthe soft magnetic underlayer, it is necessary to thin the main pole ofthe magnetic head or raise the saturation magnetization (Bs) of the softmagnetic underlayer significantly. Nevertheless, when the recordinglayer of the medium has a relatively high coercivity (Hc), the intensityof write magnetic field must be increased, and the thinning of the mainpole produces a problem of main pole saturation.

Furthermore, there are other problems including that no material hasbeen found which can increase the saturation magnetization of softmagnetic underlayers considerably.

SUMMARY OF THE INVENTION

In view of such problems in the conventional art, it is an object of thepresent invention to provide a perpendicular magnetic recording systemin which a soft magnetic underlayer of a double layered perpendicularmagnetic recording medium is prevented from saturation while the softmagnetic underlayer is designed in a thickness smaller than heretofore.

The present inventor has found that the track width of a recording headand the effect of the track ends can be incorporated into the design ofthickness of a soft magnetic underlayer to reduce the soft magneticunderlayer in thickness when the track width is small. With reference toFIGS. 1 and 2, description will be given of the relational expressionswhich the present inventors have found in connection with thethicknesses of soft magnetic underlayers of double layered perpendicularmagnetic recording media.

FIG. 1 is a schematic diagram of the top of a recording head as seenfrom the floating surface, or a diagram explaining the magnetic fluxinside a soft magnetic underlayer. Magnetic flux 3 extending from a mainpole of a magnetic head for recording magnetization transitions on amagnetic recording medium passes through a soft magnetic underlayer ofthe magnetic recording medium to return to an auxiliary pole 2 of themagnetic head. The paths through which the magnetic flux 3 flows can beclassified into paths A-C. The path A runs at the track center, nearlystraight across the opposed faces of the main pole 1 and the auxiliarypole 2. The paths B return from the sides of the main pole 1 to theauxiliary pole 2. The paths C return to the auxiliary pole 2 aroundbehind the ends of the main pole 1. Then, the main pole 1 can be dividedinto a region I at the track center, provided with the path A alone, andregions II on both sides, each provided with the three paths A, B, andC.

FIG. 2 is a schematic diagram showing the top of the main pole 1 of themagnetic head and a soft magnetic underlayer 6 of a double layeredperpendicular magnetic recording medium, or a diagram explaining thecross sections of paths through which magnetic flux flows. Assume herethat the width of each region II of the main pole 1 is W_(S). Besides,let T_(m) stand for the thickness of the main pole 1, T_(WW) the trackwidth of the main pole 1, B_(S1) the saturation flux density of the mainpole 1, T_(b1) the thickness of the soft magnetic underlayer 6, andB_(S2) the saturation flux density of the soft magnetic underlayer 6.The limit to which the soft magnetic underlayer 6 is not saturated bythe magnetic flux extending from the region I of the main pole 1 isreached when the product of the top area of the region I of the mainpole 1 and the saturation flux density B_(S1) equals to the product ofthe parallel-to-surface area of the soft magnetic underlayer 6 to makethe path A and the saturation flux density B_(S2). In other words, atthe time when the following expression holds:

B _(S2) ×T _(b1)×(T _(WW)−2W _(S))=B _(S1) ×T _(m)×(T _(WW)−2W _(S)).

The foregoing expression is arranged into the following expression (1′):

B _(S2) ×T _(b1) −B _(S1) ×T _(m)  (1′)

Like the region I, the regions II need to satisfy the relations betweenthe products of area and saturation flux density for all the three pathsA, B, and C mentioned above. More specifically, the following expressionneeds to be satisfied:

B _(S2)×(T _(b1) ×W _(S) +T _(b1) ×W _(S) ×T _(b1) ×T _(m))=B _(S1) ×T_(m) ×W _(S)

Hence

B _(S2) ×T _(b1)×(2W _(S) +T _(m))=B _(S1) ×T _(m) ×W _(s)  (2)

The expression (2) makes a sufficient condition when the distancebetween the main pale 1 and the auxiliary pole 2 is nearly equal to orgreater than the thickness T_(m) of the main pole 1, and the track widthis nearly equal to or smaller than the same. In this case, the maximumlimit of T_(WW) is twice W_(S), or

T _(WW)=2W _(S)  (3)

The expressions (2) and (3) lead to the following expression (4):

T _(b1)=(B _(S1) ×T _(m) ×T _(WW))/2(B _(S2)×(T _(m) +T _(WW)))  (4)

The expression (b 4) is based on the assumption that the magnetic fluxfrom the main pole 1 enters the soft magnetic underlayer 6 and returnsto the auxiliary pole 2 without any loss. In reality, the expression (4)is a sufficient condition, not a necessary condition. Thickening thesoft magnetic underlayer 6 more than necessary entails a number ofproblems as described above. Thus, the thickness T_(b1) of the softmagnetic underlayer 6 turns out to be sufficient if it satisfies thefollowing expression (5):

T _(b1)<(B _(S1) ×T _(m) ×T _(WW))/2(B _(S2)×(T _(m) +T _(WW)))  (5)

The calculations obtained for the sake of confirming the foregoingrelation as to the thickness T_(b1) of the soft magnetic underlayer 6are shown in FIG. 3. FIG. 3 is a graph showing the maximum intensitiesof write magnetic field obtained under the main pole of a magnetic head(the maximum values were obtained since the magnetic field under a mainpole is not completely uniform, having some distribution) while changingas the thickness T_(b1) of the soft magnetic underlayer of a magneticrecording medium, with the track width T_(WW) of the main pole of 0.4μm. Here, the ordinate is normalized with the maximum intensity of writemagnetic field with the thickness of the soft magnetic underlayer of 0.5μm. The curve indicates the normalized write field intensity which dropsfrom “1” as the soft magnetic underlayer decreases in thickness. This isascribable to the degradation of the soft magnetic underlayer because ofsaturation.

The chart shows with arrows a thickness of the soft magnetic underlayerfor K=1.0 and a thickness of the soft magnetic underlayer for K=0.25,given by the following expression:

T _(b1) =K×(B _(S1) ×T _(m) ×T _(WW))/2(B _(S2)×(T _(m) +T _(WW)))

It is seen that the normalized write field intensity becomesapproximately “1” at K=1.0, whereas the normalized write field intensitysharply decreases in the vicinity of K=0.25. Fabrication of magneticrecording media inevitably involves a certain range of thicknessvariations, and if the center value were settled in the region ofdrastic changes in field intensity, the field intensity itself wouldunfavorably make great changes due to the variations. Thus, in favor ofmild variations of the write field intensity with respect to a change inthe thickness of the soft magnetic underlayer and in terms of fieldintensity securability, the lower limit can be set to K=0.25. This showsthat the thickness of a soft magnetic underlayer needs to satisfy thefollowing expression (6), as well as the expression (5), to reduce thedependency of the intensity of write magnetic field on soft magneticunderlayers:

T _(b1) <K×(B _(S1) ×T _(m) ×T _(WW))/2(B _(S2)×(T _(m) +T _(WW)))  (6)

Where K=0.25. It is seen from FIG. 3 that the thickness Tb1 of a softmagnetic underlayer, under the above-described conditions, adequatelyranges from 0.05 to 0.2 μm. The thickness of a soft magnetic underlayerof 0.2 μm or less is significantly small as compared with the thicknessof the soft magnetic underlayers of conventional double layeredperpendicular magnetic recording media.

The expressions (5) and (6) are derived on the condition that the trackwidth be sufficiently smaller than the distance between the main pole 1and the auxiliary pole 2, or that the expression (3) hold. Besides, toosmall a distance between the main pole and another pole is inappropriatefor perpendicular recording. Moreover, when the distance between themain pole and the auxiliary pole is small, the paths along which themagnetic flux extending from the main pole will not reach the softmagnetic underlayer but return to the auxiliary pole might become minorin reluctance, in which case the perpendicular component of the writemagnetic field becomes weak. Accordingly, the distance between the mainpole and the auxiliary pole needs to be sufficiently greater than thedistance between the head top and the soft magnetic underlayer.

The numerical conditions for satisfying the expression (3) are difficultto present in general form since they involve the ratios of the main andauxiliary poles to the overall size. Nevertheless, as far as realisticconditions are concerned, the satisfaction of the expression (3)requires a distance of 0.5 μm or greater between the main and auxiliarypoles, considering that the track width T_(WW) needs to be 0.5 μm orsmaller to achieve a high recording density equivalent to or higher thanexisting ones, and that the main pole actually has a thick T_(m) of theorder of 0.5 μm. That is, the effect of the track ends of the main poletypically comes into play to satisfy the expression (3) when the trackwidth T_(WW) is smaller than or equal to 0.5 μm and the distance betweenthe main and auxiliary poles is greater than or equal to 0.5 μm. Here,the soft magnetic underlayer of the perpendicular magnetic recordingmedium may be set to, for example, 0.2 μm or smaller in thicknessT_(b1).

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by like reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram explaining the magnetic flux inside a soft magneticunderlayer;

FIG. 2 is a diagram explaining paths through which magnetic flux flows;

FIG. 3 is a chart showing the simulation results which corroborate thepresent invention;

FIG. 4 is a diagram explaining the cross sections of the paths throughwhich magnetic flux flows on a conventional model;

FIG. 5 is a diagram explaining the structures of a double layeredperpendicular magnetic recording medium and a recording head;

FIG. 6 is a diagram explaining the magnetic flux in the double layeredperpendicular magnetic recording medium and the recording head;

FIG. 7 is a chart showing the relation between a track width and thethickness of a soft magnetic underlayer in the present invention;

FIG. 8 is a diagram showing another example of the structure of therecording head to which the present invention is applicable;

FIG. 9 is a schematic sectional view showing another structural exampleof the double layered perpendicular magnetic recording medium; and

FIG. 10 is a schematic sectional view showing another structural exampleof the double layered perpendicular magnetic recording medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, description will be given of an embodiment of the presentinvention.

FIG. 5 is an enlarged view of a magnetic head and magnetic recordingmedium in a perpendicular magnetic recording system according to thepresent invention. The magnetic recording medium is a double layeredperpendicular magnetic recording medium having a soft magneticunderlayer 6 and a recording layer 5 on its substrate 9. A coil 4 iswound around a main pole 1 of a recording head. A write current flowingthrough the coil 4 induces write field. The recording head also has anauxiliary pole 2 magnetically coupled to the main pole 1, and isprovided with a read element 8 between the auxiliary pole 2 and a lowershield 7 opposed thereto.

FIG. 6 is a diagram explaining the magnetic flux in the double layeredperpendicular magnetic recording medium and the recording head. As shownin FIG. 6, the magnetic recording medium moves in the direction of thearrow, magnetization transitions being recorded onto the recording layer5 under the main pole 1. The region past the main pole 1 makes theregion after recorded, and the portion ahead of the mail pole 1 is theregion before recorded.

In the present embodiment, the main pole 1 has a track width T_(WW) of0.4 μm, a thickness T_(m) of 0.5 μm, and a saturation flux densityB_(S1) of 1.6 T. The auxiliary pole 2 has a width of 20 μm, a thicknessof 3 μm, and a saturation flux density B_(S) of 1.0 T. The distanceG_(1w) between the main pole 1 and the auxiliary pole 2 is 5.0 μm. Themagnetic recording medium uses a CoCrPt ternary type material of 25 nmin thickness for the recording layer 5, with a coercivity ofapproximately 3000 Oe. The soft magnetic underlayer 6 is made of CoNbZr,with a thickness T_(b1) of 0.01 μm and a saturation flux density B_(S2)of 1.2 T.

FIG. 7 shows the upper limit of the thickness of a soft magneticunderlayer with respect to a track width, corresponding to theexpression (4), and the lower limit of the same corresponding to theexpression (6), along with the position of the present embodiment. Inthe present embodiment,

B _(S1) ×T _(m) ×T _(WW)/2(B _(S2)×(T _(m) +T _(WW)))=0.15 [μm],

and thus the thickness T_(b1) of the soft magnetic underlayer satisfies

0.15×0.25 [μm]<T _(b1)=0.1 [μm]<0.15 [μm],

falling in between the upper and lower limits. Here, the intensity ofwrite magnetic field under the auxiliary pole is approximately 6000 Oe.The intensity of write magnetic field reaches approximately twice thecoercivity, thereby offering favorable overwrite characteristics.

The recording head described in the present embodiment has a two-polestructure consisting of a main pole and an auxiliary pole opposedthereto. Nevertheless, the present invention is applicable to anyrecording head pole structure which consists of a primary pole orprimary poles for actual magnetic recording and a secondary pole orsecondary poles with a flux-returning function but not for recording,including the pole structure consisting of three poles, a main polebetween two auxiliary poles, as shown in FIG. 8.

Besides, the double layered perpendicular magnetic recording mediumdescribed in the present embodiment has the recording layer arrangeddirectly on the soft magnetic underlayer (soft magnetic layer). However,the present invention is applicable to a double layered perpendicularmagnetic recording medium in which, as FIG. 9 schematically shows incross section, a Ti or other intermediate layer for controlling theorientability of the recording layer is arranged on the soft magneticunderlayer. Moreover, the present invention is also applicable to adouble layered perpendicular magnetic recording medium in which, as FIG.10 schematically shows in cross section, an SmCo or other magnetic layerserving as a magnetic pinning layer for magnetic domain control isarranged under the soft magnetic underlayer. An NiAlP substrate, a glasssubstrate, and the like may be used as the substrate of the doublelayered perpendicular magnetic recording medium. Carbon or othermaterial may be used for the protective layer. The soft magneticunderlayer may be formed of NiFe, FeAlSi, and the like, aside from theabove-mentioned CoNbZr. The recording layer may be formed of CoCrTa,CoCrTaPt, FePt, and the like, aside from the above-mentioned CoCrPt.

According to the present invention, the track width of the recordinghead can be incorporated into the design of thickness of the softmagnetic underlayer to reduce the soft magnetic underlayer in thicknesswhen the track width is small. This permits a reduction in thefabrication time and in the cost of a perpendicular magnetic recordingsystem that uses a double layered perpendicular magnetic recordingmedium. In addition, the medium improves in surface roughness, therebyallowing a reduction of the head's flying height to facilitate theenhancement of recording density.

While there has been described what is at present considered to be apreferred embodiment of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A perpendicular magnetic recording systemcomprising a perpendicular magnetic recording medium having a softmagnetic underlayer and a magnetic recording head for performingmagnetic recording on said perpendicular magnetic recording medium, saidmagnetic recording head having a plurality of poles including a mainpole for finally recording a magnetization reversal on saidperpendicular magnetic recording medium, the perpendicular magneticrecording system satisfying T _(b1)<(B _(S1) ×T _(m) ×T _(WW))/2(B_(S2)×(T _(m) +T _(WW))), where T_(b1) is the thickness of said softmagnetic underlayer in said perpendicular magnetic recording medium,B_(S2) is the saturation flux density of the same, T_(m) is thethickness of said main pole along a track direction in the vicinity ofits floating surface, T_(WW) is the track width of the same and is lessthan 0.5 μm., and B_(S1) is the saturation flux density of the same. 2.The perpendicular magnetic recording system according to claim 1,wherein the thickness T_(B1) of said soft magnetic underlayer satisfiesT _(b1)>0.25(B _(S1) ×T _(m) ×T _(WW))/2(B _(S2)×(T _(m) T+T _(WW))). 3.The perpendicular magnetic recording system according to claim 1,wherein the distance between said main pole and another pole of saidmagnetic recording head is greater than or equal to 0.5 μm.
 4. Theperpendicular magnetic recording system according to claim 2, whereinthe distance between said main pole and another pole of said magneticrecording head is greater than or equal to 0.5 μm.
 5. The perpendicularmagnetic recording system according to claim 3, wherein the thicknessT_(b1) of said soft magnetic underlayer in said perpendicular magneticrecording medium is smaller than or equal to 0.2 μm.
 6. Theperpendicular magnetic recording system according to claim 4, whereinthe thickness T_(b1) of said soft magnetic underlayer in saidperpendicular magnetic recording medium is smaller than or equal to 0.2μm.